One type of uniaxial tension focus mask has a plurality of spaced-apart first metal strands located adjacent to an effective picture area of the screen of the CRT. The spacing between the first metal strands defines a plurality of slots substantially parallel to the phosphor lines of the screen. Each of the first metal strands, across the effective picture area of the screen, has a substantially continuous first insulator layer on a screen-facing side thereof. A second insulator layer overlies the first insulator layer. A plurality of second metal strands are oriented substantially perpendicular to the first metal strands and are bonded thereto by the second insulator layer. During operation of the uniaxial tension focus mask two different potentials are applied to the first and second metal strands to focus the electron beams transmitted through the slots in the focus mask.
A method of making the uniaxial tension focus mask includes prodding, e.g., by conventional spraying, a first coating of an insulative, devitrifying solder glass onto the screen-facing side of the first metal strands of a mask sheet. A suitable solvent and an acrylic binder are mixed with the devitrifying solder glass to give the first coating a modest degree of mechanical strength. A devitrifying solder glass is one that melts at a specific temperature to form a crystallized glass insulator. The resultant crystallized glass insulator is stable and will not remelt when reheated to the same temperature. The first coating has a thickness of about 0.14 mm. A frame, to which the mask sheet is attached, is placed into an oven and the first coating is dried at a temperature of about 80.degree. C. After drying, the first coating is contoured so that it is shielded by the first metal strands to prevent the electron beams, passing thought the slots, from impinging upon the insulator and charging it. The contouring is performed on the first coating by abrading or otherwise removing any of the solder glass material of the first coating that extends beyond the edge of the first metal strands and would be contacted by either the deflected or undeflected electron beams. The frame with the mask sheet attached thereto is placed into an oven and heated in air. The structure comprising the frame and mask sheet are heated over a period of 30 minutes, to a temperature of 300.degree. C. and held at 300.degree. C., for 20 minutes. Then, over a period of 20 minutes, the temperature of the oven is increased to 460.degree. C. and held at that temperature for one hour, to melt and crystallize the first coating to form a first insulator layer on the first metal strands. The resultant first insulator layer is only substantially continuous; i.e., the layer includes voids caused by baking out the binder and the solvent that are used to deposit the conventionally sprayed first coating. The first insulator layer, after firing, has a thickness within the range of 0.5 to 0.9 mm (2 to 3.5 mils) across each of the first metal strands. Next, a second coating of a suitable insulative material, mixed with a solvent, is applied, e.g., by conventional spraying, to the first insulator layer. Preferably, the second coating is a non-devitrifying (i.e., vitreous) solder glass. Vitreous solder glass is preferred for the second coating because when it melts, it will fill the aforementioned voids in the surface of the first insulator layer without adversely affecting the electrical and mechanical characteristics of the first insulator layer. Alternatively, a devitrifying solder glass may be used to form the second coating. The second coating is applied to a thickness of about 0.025 to 0.05 mm (1 to 2 mils). The second coating is dried at a temperature of 80.degree. C. and contoured, as previously described, to remove any excess material that could be struck by the electron beams. Subsequently, second metal strands are applied to overlie the second coating of insulative material. The second metal strands are substantially perpendicular to the first metal strands.
The conventional spraying steps for the first and second insulator layers require the mixing of a dry glass material with at least a suitable solvent, and usually a binder. The conventional spraying steps are wasteful of the insulative material, because most of the insulative material passes through the large slots, or openings, in the mask sheet. Additionally, the excess material cannot be salvaged, and a drying step is required, prior to the firing step that solidifies the sprayed mixture into a glass insulator layer. Furthermore, conventionally sprayed material usually adheres not only to the edges of the mask openings, but often splatters onto the oppositely disposed gun-facing surface of the first metal strands; the material must be removed therefrom and from the edge of the openings, to prevent charging of the insulator by the electron beams.
It is desirable to eliminate the above-described waste of insulative material and to decrease the time required to make the uniaxial tension focus mask by simplifying the steps of applying the insulative material to form the insulator on the mask sheet. Also, it is desirable to provide a first insulator that does not contain voids, in order to increase the electrical integrity of the uniaxial tension focus mask.